The present invention relates generally to magnetic bearings and more particularly to a new method and an electronic circuit for controlling the energization of the different electrical coils which govern the generation of a magnetic field capable of varying the total magnetic flux in the magnetic bearing.
A particular type of magnetic bearing which the invention is concerned with is a device comprising a stator and a rotor member positioned coaxially with each other in the same plane without mechanical contact therebetween. The stator member or the rotor member include a permanent magnet the magnetic flux of which ensures the axial alignment of the rotor member relative to the stator member. Also the stator includes four segmental electrical coils, two of said coils being centered on each coordinate axis in the common plane of the stator and rotor members. These segmental coils are effective, in a manner known per se, to generate in the annular gap between the stator and rotor members, magnetic fields which serve to control the radial position of the rotor member relative to the stator member along the two coordinate axes in their common plane.
For that purpose, the opposed coils which control the radial position along either axis are usually connected in series or in parallel arrangement in a control loop. When the rotor is moved away from its radial position centered on the X-axis for instance, the deviation signal is electronically processed so as to cause a current to flow in the two coils associated to said X-axis, the current flowing in the positive or negative direction depending on the polarity of the deviation signal. The resulting magnetic fields thus produced by the coils are such that the permanent magnetic flux is strengthened in the larger gap and weakened in the smaller gap. The rotor thus experiences a higher attraction force on the coil side where the gap is larger than on the opposed coil and is thus restored to a position of concentricity.
When considered for space applications the aforedescribed device is characterized by the following disadvantageous features:
(a) the amplifiers driving the coils must be capable of supplying bi-directional currents and this requires the availability of a symmetrical voltage power source which usually is not available on satellites, or an amplifier with a bridge-type output stage which results in relatively complex circuitry; PA0 (b) the amplifiers used are linear amplifiers which dissipate substantial power, typically in the order of several tens of watts in their output stages; PA0 (c) all coils on a given control axis are energized simultaneously, which results in a waste of power in the coils under conditions of high rotor eccentricity, e.g. during lift-off.